Recently, CO2 reduction has been strongly required for global environmental protection, so that technologies to fuel-efficient have been vigorously developed in the automotive field. Such technologies to fuel-efficient are mainly related to hybrid vehicles and electric vehicles, which will be rapidly becoming popular in the future. Hybrid vehicles and electric vehicles include an electric motor and a generator and are partly or entirely driven by the electric motor. An oil-cooling type can be employed to cool the electric motor in hybrid vehicles or electric vehicles. In this case, typical automatic transmission fluid (ATF) or continuously variable transmission fluid (CVTF) is usually usable as a lubricating oil composition. Such a lubricating oil composition is blended with a variety of additives so that the lubricating oil composition is provided with properties for controlling wet clutch friction and for suppressing wear between metal-metal (i.e., resistance to wear between metals), and has a volume resistivity of approximately 107 Ωm. The volume resistivity of the lubricating oil composition is lowered as the lubricating oil is deteriorated. In view of the above, the lubricating oil composition usable in hybrid vehicles or electric vehicles is required not only to be excellent in resistance to wear between metal-metal but also to be excellent in electrical insulation properties for reliably ensuring the insulation of the electric motor for a long time.
Accordingly, there has been suggested a lubricating oil composition containing a lubricating base oil, and a phosphorus compound selected from the group consisting of (A) a zinc dithiophosphate containing a hydrocarbon group, (B) a triaryl phosphate, (C) a triaryl thiophosphate and mixtures thereof, the lubricating oil composition exhibiting a volume resistivity of 1×108 Ωm or more at 80 degrees C. (see, for instance, Patent Literature 1). There has also been suggested a method of supplying a lubricating oil composition containing (a) a base oil, (b) an oil-soluble phosphorus-containing substance and (c) an anticorrosive agent to a transmission.